The invention relates to an optical reflective star device for transmitting signals between a plurality of transceivers, provided with a plurality of ports for distributing each signal supplied to one port over the other ports, comprising a plurality of star elements with a plurality of reflection branches and a plurality of through-connections between the star elements. Such a device is known from the article entitled "Reflective Single-Mode Fibre-Optic Passive Star Couplers", published in Journal of Lightwave Technology, Vol. 6, No. 3 Mar. 1988, pages 392-398.
In said article the structure of various star configurations is described, as is the advantage of lower losses in the star configuration than in the bus configuration. The use of the star configuration in LAN networks is therefore becoming increasingly more frequent.
Moreover, in said article preference is given to reflective star devices compared to transmissive star devices, since in the case of the former devices the number of glass fibres and consequently the number of the associated components is lower by a factor of 2 compared to the transmissive star device. Instead of separate inputs and outputs for the transmitter and receiver device, the reflective star device has ports each serving for both inputting signals and outputting them from the reflective star device from a transmitter and receiver device. The various connections could then be provided with diplexers for the separation of transmitted and received signals.
The known reflective star device is composed of a number of star elements, for example 3-dB coupling devices, transmissive star elements or reflective star elements which distribute a signal supplied to the port of such a star element over its other ports. Reflectors are connected to a number of ports of the elements to create reflective sites, while the remaining ports of a star element are connected to ports of another element by means of through-connections.
A reflective star device can be used in a network for transmitting optical signals between a number of transceivers. Because of losses in the network, for example due to attenuation, splitting or a combination of signals, it is often desirable to make use of amplification of the signals which are transmitted between the transceivers.
Optical amplifiers have limited output power. Connecting an amplifier directly downstream of a transmitter, where the power is, after all, already high, does not therefore always make sense. In addition safety problems are associated with very high power.
Optical amplifiers introduce additional noise. Connecting an amplifier upstream of e receiver does not therefore always make sense since the useful signal is difficult to distinguish from the noise.
The object of the invention is to provide a reflective device of the type mentioned above, in which the problems mentioned above are overcome.
This object is achieved according to the invention in that an optical amplifier is disposed in at least one of the reflection branches and/or through-connections.
A reflective star device, from an optical point of view, is perfectly symmetrical and is therefore nearly always arranged centrally in an optical network. Thus, the reflective star device will always be in the centre of the transmission section between the sender and the transmitter of the transceivers, as a result of which amplification will always take place in the centre of the transmission section. In this way sufficient signal strength can be achieved for noise to be no longer significant. However, the signal is not so strong as to be able to cause the optical amplifier to be overdriven.
Furthermore, the invention has the advantage that, as a result of using amplification in the reflective star device itself, considerably fewer amplifiers are required, which is of increasing importance in particular for fairly large networks.
Apart from that, in the case of optical amplifiers, half the amplification factor in one direction is sufficient in the reflection branches, since the optical signal passes through the amplifier twice when reflected.
It is observed that from "Patent Abstracts of Japan", vol. 14, no 343 (P-1082) Jul. 25, 1990 & JP, A2 120 830 (NEC), May 8, 1990 a distribution network having a number of reflection branches with reflective amplifiers is known per se. Said known network differs from a reflective star. Moreover, the amplifiers in the known network cannot operate concurrently. One is in operation and if this one fails, another amplifier is switched on. Furthermore, a through connection provided with an amplifier is not known from said "Patent Abstracts of Japan"publication.
In one embodiment of the invention, the optical amplifier disposed in a through-connection is a bidirectionally pumped optical glass-fibre amplifier, for example an erbium-doped optical fibre amplifier.
In another embodiment of the invention, the amplifier disposed in the reflection branch is an optical semiconductor amplifier, preferably combined with a polarisation-independent reflector.